Best boiler replacement

GW

yes, I'm sure it's done daily, but the nice man was being lead to beleive he's going to realize 30% savings if it's done right? I'm not seeing it, help me understand. If this guy want's 90* air blowing by his toes, all the power to him.

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rumn8r

Best boiler replacement

I am planning on replacing my current hydronic forced air heating system which is nearly 40 years old. I was considering a mod-con boiler but venting is quite difficult with several $$$ options. The HVAC contractor suggested that a ~85% non condensing boiler is probably a better option which can use the current chimney. He said that initial cost is less, reliability is better, installation is less (because of the venting issue), and that we only lose ~7-8% in efficiency. He further suggested that 2 smaller boilers (175 K btu each) would provide redundancy if we were not at home for extended periods in the future (after retirement, not in the foreseeable future unfortunately). This is to provide hot water to hydronic coils in 2 variable speed air handlers (Carrier or Trane), indirect water heater, swimming pool heat exchanger, and another hydronic coil in a Desert Aire unit for the pool room. I guess it makes sense to go with a conventional boiler rather than a mod con because of the venting and reliability issues. What about 2 boilers vs. one? What boilers should I consider?

Josh_10

This would be my advice.. Mod/Con boilers vent with PVC so I am curious as to why it is more difficult to vent. Mod/Con's are quite reliable. I am not sure what your contractor is getting at there. And there is a major savings over conventional boilers which by all means is 50 year old technology.

I would get a second opinion. That is my advice.

Ragu_5

I Second The Motion...

Get 3 opinions. Find a Contractor from this site for starters.

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rumn8r

I already have a half a dozen "local" opinions and have hired a ME for a complete plan. I found 2 local HVAC contractors that I could work with. Closest pro on this site is ~ 70 miles away, a couple more ~100 miles away. Venting condensing boilers/furnaces is an issue, requiring insulated outside runs and masonry work. I would appreciate opinions on 2 vs. 1 unit and the best non condensing boiler for this purpose. Thanks.

Perry_2

Hmmm

I am another homeowner who also looked at the "old fashion" boilers and the mod/cons.

You are of course free to do what you wish. Concerning good old fashion boilers - their are a variety of brands.

But - the thing that I found was that in my area none of the Heating contractors knew how the mod/cons "really" worked - or even how to adapt them to my system. Thus, they recommeded the older boilers they knew so well.

You can buy a 87% efficiency boiler - and a 94% efficient mod con. The difference you get is not a 7% improvement in efficiency - more like a 30% improvement in efficiency for the same sized boiler - if the mod/con is hooked up and controlled right.

The mod/con operates much different than the old boiler system in most applications. It seem not all understand that.

In the end - I chose to get a Viessmann Vitodens 200 mod/con - and worked with the Mfr rep to ensure I understood how it worked and how it needed to be controlled. I then found a heating contractor who was willing to work with me and install it.

I started out by specifying the older boilers to the bidders myself. A little education showed me that it was time to move ahead of where the local Heating Contractors were comfortable with.

Perry

Dave_4

Perry,

Thanks for sharing how you went about things. I have considered Viessmann Vitola 200 as a modulating non-condensing boiler and would consider the Vitodens 200 in a mod con. One contractor I liked has experience with mod con boilers and first suggested the Lochinvar Knight until he studied the venting options; he then recommended that I go with a conventional cast iron boiler. If I go with a mod con, one concern that I have with teh Vitodens 200 is the limited vent length (20' I think) and I also wonder about availability of parts in Michigan. Also, I read that the low air temperature of hydronic forced air systems using modulating boilers feels drafty which bothers some people. Lastly, can you explain how a 93% efficient mod con boiler can be 30% more efficient than a 85% efficient conventional boiler.

Perry_2

Sure thing

As for venting: I moved the boiler to the side of the house - and vented it out the side (and had to vent up into a first floor closet, and then out the side of the house to get 1 Ft above the snowline).

I then piped the hot water to where I could connect to the existing heating piping.

The mod con - controlled with an outdoor reset and other features as appropriate to your application works by running as much as possible - and extracting the extra energy from the exhaust gases as practical. The boiler is actually more efficient at the lower loads (and 98% is achievable for minimum firing for some mod/cons).

An older type furnace fires up, and shuts down, a lot. While the furnace may be 87% efficient - that is typically the efficiency at maximum firing rate after the boiler has warmed up - modulating conventional boilers typically operate at lessor efficiencies at lower firing rates. Also - A lot of energy is lost warming up the boiler every time it starts up - and this is not considered in the boiler "efficiency" numbers.

It is also inherently more efficient to heat water to 100 F than 140F or 180F (whatever your system was designed for).

The system differs in operation (my ciruculating pump runs almost all the time - circulating warm water (even when the water is only 85F) instead of running for a bit with 140 F water - and then allowing everything to cool down to near room temperature again before the boiler fires again.

So, not only do you get a more effiecient boiler - but you get a much more efficient way to operate the boiler and the system. Hence the comparitively large efficiency gains.

I do admit I missed the "warm" radiators of the old method at first - but after a couple of weeks I've adjusted. The system I installed holds my house rock steady in house temperature.

Hope that helps.

ps: Where in Michagan. Steve Ebels, who frequents this forum, is from Michagan and installs Vitodens (he helped me with some of my questions). You might wish to have him take a look if he is in your neck of the woods.

Perry

Perry

Scott04

Mod cons and hydro air

Let me start by saying that I am not familiar with mod con units. What I am going to post are simply some of my thoughts, in hopes that I can get a better understanding. Please do not take what I am going to say as fact.

It is my understanding tha Mod con is most efficient at lower water temps. It seems that with hydro air the water temps would have to be more in the conventional range to produce warm enough air so it didn't feel cold coming out. If this is true, and you need 160*-180* water, wouldn't the mod con be operating more in line with a conventional boiler?

As far as two boilers vs one, if each boiler is sized to a part of the heat load, then it makes sense. That way, when the temp is not at the design temp (which is most of the time) you are more closely firing to the load. It also adds at least partial redundancy, it may not heat the house to temp on a design day, but will prevent it from freezing. This only, I would say, apply to conventional boilers, since the mod cons only supply the heat needed anyways.

Scott

Brad White_141

ModCons and Hydro Air

As for temperatures, Scott, "it depends". Siggy and others state a minimum entering water temperature of 130 degrees F. for the reasons you mention (not blowing cooler air). The "conventional" 180 degree range is not absolute but a manufacturer's rating default.

Hydronic coil selections can be had using multiple rows (up to four and even six) which can allow the use of lower temperatures. You can even get a 30 or 40 degree temperature drop out of the water and return in the 90-100 degree range while giving out 110-115 degree air with counterflow.

Even at 130 F. entering water, though, you are just at or below the dewpoint of flue gas on the supply side and certainly below it on the return side, so you will be condensing.

Bottom line though is, the air supplied at whatever temperature has to be sufficient in quantity, so expect to move more air slightly cooler. Distribution is key in this case to avoid drafts.

My $0.02

Brad

Uni R_2

rumn8r - Vito Venting Length

If you can vent vertically with the Vitodens, you can go 33' equiv. using the Viessmann coax and 66' equiv. using single wall AL-29-4C stainless (if you don't mind the air intake opening being on top of the boiler).

GW

hold that thought

You have hydro air? Take all the advice you just received and hang on for one minute... you are absolutely correct! There is no way these mod/con boilers will operate at their rated efficiencies because your fan coils need toasty hot water to be reasonable comfortable. The combustion will be above condensing conditions, which gets you nothing that a cast iron boiler won't do.

I think it's crazy to go mod/con especially when you're saying you have to pay extra to make the venting happen.

The 30% comes from some basic engineering principles: if you can lower the system temperature 3*, you can save 1% in fuel. Lower it 90*, and there you have your 30% savings. This is all fictitious mind you; with your fan coils I don't think you'll come close UNLESS your current system is a real hog. It's also a supposed average. In the mild weather the mod/com will have high efficiencies; in the deep freeze days/nights you won't be in the 90's, maybe high 80s.

The ultimate system is a new boiler operating off of outdoor reset with constant circulation to medium mass heat emitters such as cast iron rads or Euro panel rads. Well, a full blown radiant system is even better, but people usually can't drop that kind of cash. We claim 30% all the time when we convert people’s entire systems. If you have a heating guy claiming 30% savings with air handlers, nicely ask him to guarantee it!

Two boilers? I dig that idea because one can be the work horse during regular winter conditions and the second can come on during real cold conditions. It does give you slightly better overall efficiency and the security of knowing there’s two beasts down there and not just one. Sweet.

Gary


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Brad White_141

Hold that thought too

Gary- See my post below. You indeed can work Hydro Air with a mod-con, with the right air handler coil.... done all the time. Hot water for hydro air is conventional sure, but it can be done with much lower temperatures.

GW

Perry

Man, you're the smartest non-heating guy I've ever come across, you know this stuff better than most heating guys!

The application here is just a bit different though.

I don't want to be a broken spoke, but those combustion efficiencies are man made numbers; I realize 99.9% of the world abides by them. I've even heard some fruit cakes mention numbers in excess of 100%... now that's a real winner, a boiler that creates more energy than you put into it!

To get the system to work like you're describing you need constant circulation with low temp design. This guy has air handlers, way different animal.


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rumn8r

Uni R

What if it is a combination of horizontal (~12') and vertical (~12') w/ 2 90° elbows?

Perry_2

To sum up..

Let's keep this simple:

A mod con would work well IF (and only if) your air coils are suitably oversized. You are then not likely to see the full savings typical of a more normal hot water heating system - but would see more savings than a normal boiler.

If you get a more normal boiler - their are many good brands of cast iron boilers; so you are OK to deal with whatever the local heating company normally uses (and usually they will tell you that they have been installing that line for 20+ years with no major problems).

One caution on the mod/cons. The market is being flooded with many different ones. None of which have been arround for 10 years +. While their are some excellent designs from long term very solid companies - their are also designs from companies that did not exist 5 years ago. Be carefull shopping if you are going this route. I personally chose the Viessmann Vitodens after a lot of research. I kinda view it as the best of the best (Viessmann's philosophy as best as I can tell is more towards building a very good boiler regardless of the cost - and then charging for it; whereas some other companies focus on building the cheapest boiler they can that "should" last a few years).

If I were you - I'd want a good heat loss done on the house - then determine how oversized are your coils (also were they designed to operate at 140 F, 160 F, or 180 F water).

Once you know that - then people here should be able to better tell you how suitable a mod con is.

Have a great day - and remember to vote.

Perry

Uni R_2

Call Viessmann

Have you tried calling the regional Viessmann rep? That'd be a good first step in this case.

Joannie_11

Also

Don't forget the "mod" in that little phrase "mod-con". It's not just condensing that gets you more efficiency. It's also the modulating gas systems on these units. If someone wants to go with a "conventional" unit, at least get something that is either step-fired or modulating. That is not an advantage that one should overlook.

rumn8r

mod or step-fired

Which conventional boilers are modulating or step-fired?

GW

test bench

Has anyone ever proven in real life that the mod/con is this wonderful? Or do we just believe the brochure?

How about air conditioning? Do you really thing a Trane 19i delivers an EER of 19? Heavens no, the AFUE formula is like 6 million off the wall factors to fudge Up the real life number. But how many sales people toot the horn about the 19 seer being 19 seer? Well, because Trane says it is, that's why.

Back to heating. For instance, what is the oxygen level at these low temps? Have you ever seen a 90plus gas furnace run o2 levels below 10%? Not me. That tells me the combustion is like warm. That’s why I like oil, getting o2 down to sub-5 percentage is easily attainable, meaning hot combustion. Does anyone commenting on this subject have a clue what I’m talking about? The fictitious numbers a combustion analyzer can be misleading; if you breathe heavily on your combustion analyzer with can get high 90s for efficiency. If you stick a few candles in the combustion chamber, you can get high 90's because the flue temp is so low. Has anyone ever measured gas going in (versus) heat going out? It's a real pain because it's not easy to measure gpm (oh sure, we can guess like we do everything else), but what's the real gpm x the delta T (easy to read) x 500? Does it really equate to 90% of the cubic feet of gas going in? I would love it if someone out there has actually gone through these steps; the propaganda is killing me here!

Gary


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Jack Waller

Hydro Air

I don't think anyone mentioned the fact that one can operate a series loop with the higher temp emitters first. In my case I have some radiant that insures a lower return temp for condensing. However I know from having a well insulated lower level and a higher load main level, that I can take advantage of this in the way I have piped the two fan coils - hotter to the higher demand where my large windows are. As others have said there is absolutely no reason why a lower demand coil can't operate at 140 and below provided it is sized accordingly and thereby insure condensing operation at least most of the time.

And what's the problem if a few times a year you have to crank up because it's 30 below outside.

Another aspect I hope to investigate is to speed control the fan for those times when air temperature is marginal which I would judge from my experience to be about 100. This only appears to be an issue when outdoor temperatures are higher and in the reset range. High velocity 80 degree air is not pleasant for sure when you stand over it.

Just some of my thoughts, and I love my TI150 thus far ALTHOUGH I have to concede that its not made in the USA (:>)

Jack

Mark Hunt

I've seen it Gary


Customers with HWBB realizing 30% to 40% reductions in their fuel usage after we installed a mod/con. Lots of contractors in my area do not believe that mod/cons will work with systems that were supposed to be high temp. Well, those systems are only supposed to be high temp on or near design day. The rest of the time, the HWBB works just fine at MUCH lower temps.

As for the O2 readings, always below 6% and most are between 3% and 5%. I do not pay attention to the "efficiency" readings on my digital analyzers since they are based on "assumed" numbers. What I do pay attention to is the feed back I get from my customers.

I just finished installing a HWBB system in a home and it is powered by a Buderus GB142 with the AM10 OD reset control. The other day with an OD temp of 34 the boiler target temp was 112. The house stats were set to 68 and they all satisfied. I purposely over radiated the home so that I could use lower water temps. In MOST of the homes I visit with existing bsbd, they are over radiated as well. Not on purpose, no, quite by accident. But they are still over radiated and the lower temps work for the majority of the heating season.

I am starting a solar home soon that will have slab radiant for the first floor and a hydro-air system for a few rooms on the second. The hydro system is designed to heat the space with 135 degree water.

It can be done.

Mark H



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rumn8r

Perry, I'm just trying to understand...

So, oversized coils = greater delta temperature = greater water temperature from boiler, yes or no? Can you do the same thing with a controller to change the minimum water temp allowed even with a smaller drop in temp across the coil?

GW

cool

I never meant to say it couldn't be done, but often times we hear these Cinderella stories about fantastic fuel saving and put a halo over the boiler. I bet every manufacturer has a least a few Cinderella stories on how much money their unit saved the customer. The typical scenario is someone is yanking an old boiler and installing new. The numbers really get terrific when the old unit was running like a real dog.

I'm sure I’ve got some people out there thinking I'm whacked. That's fine, I like it that way.

We do a fair amount of 85% CI boilers on outdoor reset and indoor sensor/or TRV. That being said we also do a fair amount of modcom/.

We strive for 100% constant circ, no shutting down the heat unless DHW or WWSD. So, where are the stand by losses? There are none. But some people are convinced the mod/con has some magical power. Yes, I see the technology, but I guess I'm not so blown away, that is until I set up a test bench and see it with my own eyes.


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Perry_2

Not exactly

One thing that you have to watch is the return water temperature on a cast iron boiler. They get into trouble if it is too low. While different people say different things - normal return temperatures below about 110 F may lead to problems.

With a mod/con boiler you can typically have very low return temperatures without affecting the boiler - because the boiler is not cast iron.

Also, if you have oversized coils you can probably use lower inlet temperature water. Let us say that your coils were "designed" for 180 F water. If they are sufficiently oversized you may be able to run them at 140 F water (or potentially even lower than that - I believe Brad mentioned 130 F). The water return to the boiler might be 90 - 100 F - which a mod/con can easily handle.

I am aware that their are some conventional style boilers that have special perwarming chambers for incoming water to prevent issues. I am not sure how that affects overall efficiency - althoug I suspect it doesn't help.

Bottom line: How oversized are your coils - and what were their intial design temperature. If you wish to maximize long term savings you need to know.

I do admit, that many people just give up on the quest for great performance and great efficiency and just install something that will "work." It is your call where you exit this discussion.

Can you get someone who is knowlegable about these issues to visit - even if you have to pay a "consulting fee" and perhaps transportation. It might be the best investment you ever did.

Perry

GW

close

>>>>So, oversized coils = greater delta temperature (YES)= greater water temperature from boiler, yes or no (The gist is the more BTU you can "unload" the lower the temps can be. If you can't "unload" more than normal [regular sized coils], you need "regular" hot water temps. The ability to unload is directly related to the difference in temperature between the return air from the living space and the temperature of the coil, however if the surface area of the hot water coil is "increased", you technically are transferring more energy thereby having the ability to lower the water temps.)

>>>Can you do the same thing with a controller to change the minimum water temp allowed even with a smaller drop in temp across the coil? (My brain can't process that one, but I'll answer it anyway: if you lower the Delta T across the coil, you lower the BTU [unless you increase fan speed, but that's not what you're asking at this moment) If you raise the minimum water temp, then we went a full circle and you're getting away from the benefits of the modcon.)

I hope this helps!

Gary


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Brad White_142

Distribution

I am not saying just do it for it's own sake but to do it right. Don't blow 90 degree air across your toes necessarily, but up the walls and return opposite. Design it as a system, not just plug in a premium boiler to a tract house hot air system.

Done right, I think the savings will be much better, not as good as radiant but far better than using above-condensing boilers.

That is my approach!

Cheers,

Brad

rumn8r

Perry and Gary

So boilers that allow lower return air temps can be used with larger hydronic coils for greater extraction of heat, providing warmer air temps at lower boiler temps?

If so then a non mod con boiler that allows very low return air temps like the Vitola could provide the same advantage correct?

Jamie_5

off-cycle stack losses?

Amateur opinion follows, tread carefully.

I think the idea of striving for constant circulation has a lot to recommend it and I would venture to guess that the primary goal is to reduce standby losses. That is, the hope is to move as much of the the heat as is possible out of the cast iron of the boiler block and to somewhere useful in the home. That said, could you tell something about relative efficiency by measuring the draft in the combustion chamber and/or the stack temperature when the boiler is off? I am guessing that the mod/cons gain something here from being induced draft without any inducement occurring when the boiler is not firing. Low mass in the conbustion chamber would help in that regard as well, I suspect.

Perry_2

Possibly

In the end - how well the boiler will work (even a mod/con) is in the controls you get for it. I am not familiar with the contolls available for the Vitola.

I know of several mod/con installations that are being run like old fashion boilers; which buys you little.

The maximum efficiency gain is in with the proper outdoor setback curve for your specific application and with some kind of internal house temperature control as well (TRV's or something like the RS control for the Vitoden - which principally adjust which reset curve the boiler is operating on).

Keep in mind that with my reset curve, one day I am circulating 100 F water - and another (colder day) I am will be circulating 140 (or even 160 F) water. The boiler can go higher than that - but I doubt that I will ever need more than 150 F water in my application. Today, I'm circulating 85F water.

Best of luck with your project.

Perry

rumn8r

More info

The existing air handler (Trane Climatrol) is a BEAST with very high capacity (7500 cfm) but ducts and registers are sized so we don't feel drafts. This is with our 38 year old CI 300 k btu input boiler. I have observed the water supply temp to the hydronic coil range from 100° to 150° F. Water temp is modulated by outside temp with a White Rogers control unit. In any case, the replacement variable speed air handlers will have a max of 2000 and 1250 cfm and zoned with modulating dampers. I am hoping that we wont feel drafts with whatever system we end up with.

MikeL_2

Lots of good thoughts here. Let me share my practical results as a Ph.D. physicist who designed and built the heating side of the HVAC in his house 1.5 years ago.

First, the house. 4500 sq. ft. ICF house (2250 main and 2250 down) with 10 to 14 ft. ceilings. Southern exposure in SW Ontario, Canada. Heat loss (measured) is 21000 btu on design day of 14 F with a 20 mile/hr north wind.

Heating consists of a Buderus GA124/17 boiler with a Tekmar 262 controlling it. The Buderus is piped to a manifold (not P/S) that then has 2 taps off (one for DHW Amtrol indirect with a Grundfos pump and one for the air handler, also with a Grundfos pump. Both pumps controlled by the 262). The air handler is a Lifebreath unit with a large 4 layer coil and an ECM motor that runs at 350 cfm and uses 15W. The air handler motor and the circ pump run continuously unless there is a DHW call in which case the AH circ pump shuts off. The boiler is still way oversized for the space heating requirement, so I put in 40 gallon insulated buffer tank on the supply side between the GA124 and the air handler to prevent short cycling. A Tekmar 062 RTU with remote sensor and outdoor reset (on the sunny glass filled south side of the house) do the temp sensing. Lifebreath HRV handles humidity control and fresh air.

Here is the performance data from last evening/night (cloudy and 39 F for 12 straight hours, no wind).
Buffer tank target temp = 93 F (114 F on design day for your reference !).
Boiler run time = One 14 minute burn every 65 minutes.
Boiler return temp = 71 F.
House temp = 70 F +- 0.2 F
Condensate from flue trap = 0 cc.
Tekmar Differential = 25F

When the buffer tank target temo drops below 96-25/2 F point, the boiler fires up and a Honeywell A419 makes sure that the circulator is interrupted till the boiler well temp goes over 104 from it's 70 F current temperature. It's 70 F because the water temp coming back from the air handler is at room temp by this point in the cycle. Then the circulator starts up, and even though the return water from the air handler is at room temp, the boiler well never drops below 104 again. Takes about 1.5 minutes for the boiler well temp to get to 104 and then the boiler keeps running for about 10 more minutes. During this time, the boiler well temp will climb to around 140F, the buffer tank to 96+25/2 F and the return water to 86F. Then the boiler shuts off , the circulator in the air handler keeps going and the heat from the buffer tank/boiler gets drained back down and the cycle repeats itself.

I believe it's not the return water temp that matters as much as the boiler well temp. If that is over the manufacturers spec, then you should get no acidic condensate. I have never measured water to come out of the flue drain (unless I poured it in just to check it wasn't blocked and that the flue was pitched right !).

The system is in constant circ/air flow mode, so there are no standby losses. Just steady low flow air out the registers that spills out along the floor and does not shoot out and collect at the top of the ceiling, but actually keeps the floors quite pleasant. Much different than running the blower at high speed on a thermostat call for heat in the conventional way. And much more pleasant and quiet. With full humidity control, filtering and AC of course.

In this scenario the GA124 is probably doing much better than it's tested 85.2 AFUE as there are no standby losses and I'm not heating the water to 180 F. A mod-con might thrive in this situation as the return water temp, even on design day is low. But not 30% according to some numbers in this thread. And not enough to justify the additional cost (the GA124 came with the house hvac equipment package).

So it is possible to have warm floors, an extremely efficiently operated cast iron boiler, an air handler and air handler water supply temps that never exceed a mod-con's noncondensing limit. A large coil in the AH is key, as are an energy efficient pump and blower motor. A good cold start boiler will do the job. I would have put a GB142 in had it been more commonly available when we designed the house, but I don't think I'm losing out that much right now.

Next investment will be solar preheat for the air handler return (we already do it for the DHW). That's a better long term payoff than tearing out a perfectly awesome GA124 and putting in the GB142.

Dave_12

2 Boilers

If you can afford it and have the space, then two boilers are always better then one. As DH says - Belt & Suspenders. Besides the advantage of partial redundancy, two small boilers would require smaller pumps and better turndown than one large unit. While I am a fan of the mod-cons, they are not always the ideal boiler for every budget & application.

Lochinvar has their 'Solution' Boiler which is 84% eff, 2 stage firing. They have a CBN180 180,000 input unit which may be right up your alley. With a nice 4-stage control you would have ~ 4:1 turndown. These have a pumped bypass which will protect you down to about 120 F return, if you are operating below that they you will need some additional return protection.

There are a lot of good boilers out there and you probably can't go wrong with any of them. It is the installation & support that will make the best system for you.

Sounds like you are on the right path to a great system, good luck with everything.

Mike T., Swampeast MO

All of your heat consumers (hydro-air, indirect, pool heater and dehumidifier) tend to be rather "temperature hungry". The higher the temperature a mod-con provides, the lower its efficiency.

Hydro-air coils can certainly be sized for lower than typical supply temperatures, but if already installed this may not have been done. The variable-speed nature of the hydro units can definitely help, but MUCH has to do with how the fan speed is controlled. ALL mod-cons work with some sort of supply temperature reset--the lower the heating load, the lower the supply temperature. Ideally, the hydro air unit will be aware of this and adjust the fan speed in a similar fashion. By somewhat lowering the delivered volume of air with reducing supply temperature, the air temperature won't drop to the point of feeling "drafty" during periods of low heat load. Also ideally, the hydro air unit and/or its thermostat(s) will communicate back to the boiler telling it how far the current room temperature is away from the desired room temperature. This is especially important if you use daily thermostat setback and expect a quick warm-up. I haven't checked them all, but the VS hydro air units I've investigated would all require some customization to approximate these ideals and not all mod-cons would be suitable. A mod-con with an adjustable "auto boost" would likely be preferrable to one that operates on a fixed curve like the Vitodens. (Fixed once set--not utterly fixed.) Depending on the boiler and hydro-air unit used, a sophisticated Tekmar controller might be able to control everything...

350,000 btu is quite large for typical residential mod-cons. If that much heat is truly required, you're probably looking at multiple staged boilers unless you go to something like the Viessmann Vertomat with fully modulating burner ($$$$$).

Unless you can find an exceptional contractor with experience with such a system (both in size and nature), AND have a very generous budget, I'd suspect you'll be better off with at least 2 rather conventional boilers operating at conventional temperatures. Unless the system is VERY carefully engineered and VERY carefully controlled with mod-con(s), I suspect that your reduction in efficiency will not be particularly great.

rumn8r

Mike T.,

Thanks for your comments. First of all, this is in the planning stages and nothing has been purchased so I can get oversized hydro coils to get a greater temperature drop as has been discussed in this thread. I can also choose 4 speed air handlers rather than variable speed. I'm not sure how the speed is set on the Trane and Carrier air VS air handlers but I assume that there is a wide range to select from for heating and for cooling.

There seems to be disagreement within this thread on the extent to which my "heat consumers" are temperature hungry. I measured the toasty air coming from the register in my office this morning (32° F outside) and it was 106° F with my current hydro air system and the supply temp from the boiler to the hydro coil was 118° F. The air at 106° F feels quite warm and I doubt that I need to have it feel warmer than this. From what I have gleaned so far from this site a mod con will be condensing under these conditions. Maybe it won't be condensing as it gets colder. Granted, the indirect water heater is "temperature hungry" as would be the pool heater if a high temp rise is needed in a short time (heating only on weekends). The Vitodens has a built in control for indirect water heat to fire at full load. Could this be plumbed and controlled to provide "auto boost" for other zones (temperature setback)?

Even if the highest temperature curve is selected for the Vitodens, the efficiency is over 90% until the unit goes above 70% load and it is over 96% efficient when less than 40% load (see the graph on page 2 of the Vitodens Technical Data Manual). So why can't I use 2 Vitodens boilers and oversize them so that they don't approach full load? I'm probably missing something here because some have warned not to oversize a mod con boiler but I don't know why.

I doubt that I will find anyone locally with experience putting something like this together, but have found some bright folks with experience in different aspects of this (mod con boiler, vs air handlers, zoning). I'm just doing my homework up front so I know what I am getting myself into.

Mike T., Swampeast MO

rumn8r:

You're welcome. I've never used hydro-air, but I have researched it quite a bit and helped some homeowners to specify for use with a Vitodens.

As others mentioned as well, hydro-air can definitely work at lower than "standard" temperature--both the size and configuration of the HX coil must be carefully selected. As Brad mentioned, deep coils made up of more rows can extract more heat.

With a Vitodens the hydro air handler (just like the indirect) must be very carefully chosen. The maximum operating temperature for weather-responsive operation is limited to 75°C (167°F). The air handler MUST be able to provide the design load with this supply temperature. Just as important, the flow rate through the air handler to achieve this output at this temperature MUST be within reason. It's this limiting of flow that causes some people problems with the Vitodens.

Each Vitodens model has a maximum (and minimum) flow rate through the boiler HX that cannot be exceeded. The two smaller models have a built-in circulator, so this is no problem--the two larger use separate circulators, but if Viessmann recommendations are followed the max flow rate through the boiler will be correct. A system such as yours would be connected to the Vitodens via a low-loss header--its function is to hydraulically separate the boiler from the system. This prevents problems if flow through the emitter (secondary) side is either too high or too low. It also functions to maintain the lowest possible return temperature to the boiler--e.g. the exact same return temperature entering the low-loss header.

While the low-loss header isolates the boiler from high secondary flow, there can be serious problems if the high secondary flow is combined with high heat demand.

For instance: Say you have a hydro-air coil designed for 80mbh output @ 167°F supply temperature. BUT, to achieve this output at this temperature you need 10 gpm of flow. Using the simple formula to compute temperature drop (delta-t) across the HX: DT = Btu / (500 * GPM):

80000 / (500 * 10) = 16° delta-t

167° - 16° = 151° return temperature to the boiler.

Now, say the boiler is the Vitodens 8-32. The MAXIMUM permissible flow rate through this boiler is 6.2 gpm. So, the boiler must supply the 80 mbh load with no more than 6.2 gpm.

Using the same formula:

DT = 80,000 / (500 * 6.2)

Rounds to a 26°F delta-t. 151°F (the return temp for the boiler) + 26° = 178°F. BUT, the maximum temp of the boiler is 167°! What happens? [I believe] the boiler will only provide enough BTUs to heat this 6.2 GPM to 167°F (the max boiler temp)!!! The remaining 3.8 GPM of flow from the air handler blends back into this 167° supply temperature inside the low-loss header thus reducing its temperature! In a situation like this the Vitodens [may] not provide your design output! Your contractor should confirm this with Viessmann if you use the Vitodens! The Vitodens [might] be able to use an internal temperature somewhat higher than 167°F to maintain 167°F at the low-loss header--I'm not sure and it's not mentioned in the literature.

This is just the sort of thing that your contractor must consider--he can't just reduce the required supply temperature by increasing the flow rate and be guaranteed full output. Hydro-air can certainly be used and can certainly operate at temps low enough for good condensing efficiency, but you must design carefully and consider not only temperature requirement but flow rates and the action of the low-loss header.

The same things MUST be considered when sizing the HX for the pool heater!

The efficiency of ALL mod-cons reduces slightly as output increases--but ALL manufacturer's I've spoken with say you should NEVER oversize the boiler because of this. Why? Because mod-cons cannot modulate down to zero output. In any given mod-con line, the higher the maximum output, the higher the minimum output as well. Oversizing the boiler serves to reduce the available range of modulation forcing the boiler to operate in on-off conditions much of the time. This negates much of the benefit of modulation!

A mod-con is most efficient when driving a system at the lowest possible temperature to just counterbalance the amount of heat being lost. The greater the temperature drop across the boiler (e.g. the lower the return temperature) the greater the efficiency as well. With hydro-air, this means you want the hydro unit running for very long cycles whenever you need heat (well at least when the heat load is above the minimum modulation level of the boiler). This can cause a conflict if you use daily setback and expect a quick warm-up as the required supply temperature will be significantly higher than if the space temperature were constantly maintained. Without an extremely expensive BMS (building management system) and likely some custom components/modifications as well, I know of no way for the air-handler to communicate back to a Vitodens to say, "Hey! I'm trying to raise space temperature rapidly and I need a higher supply temperature." You could use the timer program built into the Vitodens. While it offers multiple events in a given day, anytime you want "normal" temperature the boiler will have to operate at a curve high enough to increase the space with the desired speed. If that "raise the space temp" curve is significantly higher than the "maintain the space temp" curve, the air handler will be turning on and off MUCH more frequently--more like a non-modulating system. You'll loose some boiler efficiency in two ways: higher supply temperatures and reduced modulation. (Modulation is reduced because the boiler is no longer maintaining space temperature--instead it's letting space temperature "bounce" around a thermostat setpoint.) Without direct, proportional communication between the air handler(s), thermostat(s) and boiler(s), I know of no way around this problem. The Tekmar TN4 system [might] be able to accomplish this with some mod-cons, but not (to my knowledge) with the Vitodens.

When multiple mod-cons are used in the same system, most (if not all) are connected by a manufacturer-supplied control/communication module. When one boiler can't meet the load, others drop in. The control/communication module will also rotate the "lead" boiler to keep the total firing time of all the boilers similar.

While I can't say for sure, it's sounding to me as if you'd be better served by using two boilers in two different systems. One providing the hydro-air and the other providing heat for the pool. Which supplies the indirect and which supplies the Desert Aire would depend... Given your operation of the pool (usually heated only on weekends), I suspect that it will need a high and generally fixed supply temperature with a high flow rate. Thus, a well-sized conventional boiler would likely be a good choice.

You could then use a mod-con for the space heating. Hopefully the Desert Aire unit can operate on the same supply curve as the hydro-air. If not, things get even more complicated as you're talking a two-temperature system with the efficiency defined by highest supply temperature and modulation rate currently being produced. If you choose a Vitodens for space heating, I would HIGHLY recommend the Viessmann indirect! Other indirects have just the same sort of problems as those described above regarding hydro-air. HXs are generally too small with required flow rates too high and flow restriction too great.

The Desert Air unit is your real wildcard here. Since I'm sure it will be used year-round, it would probably be most economical to operate it via a mod-con--BUT I would suggest that it should be operated to accept AT LEAST 1/2 of the minimum modulation rate of the mod-con. If the Desert Air input is less than half of minimum modulation, I'd say that a buffer tank is required.

Mike T., Swampeast MO

Correction To Above!!!

When secondary (emitter) flow exceeds primary (boiler) flow the Vitodens can fire to an absolute maximum INTERNAL temperature of 172°F (78°C) in an attempt to maintain 167°F (75°C) in the low-loss header.

So, there's a BIT of headroom when you need higher temperatures with high secondary flow, but not too much. When secondary flow greatly exceeds primary (certainly possible in some systems and designs) this can really haunt you and make you wonder why the boiler is not firing to maximum when you want it most...

Regardless of this temperature problem however, relatively high secondary flow will reduce boiler delta-t by increasing the return temperature. This in turn reduces efficiency due to less condensation than possible if secondary flow were more in line with primary.

rumn8r

Mike T.,

I'm getting quite an education here, thanks! Could I use a buffer tank to supply the pool heat exchanger and use one of an indirect hot water zones to supply it (the other would be for an actual indirect HW tank? I will require 2 boilers if I go with the Vitodens so I will have 2 different indirect zones to supply heat at full flame.

Brad White_9

Just an idea


but what if you were to use a different boiler for the pool and DHW? If you took those two loads off of the total requirement, you could use one Vito for the house and one sealed combustion, non-mod/con boiler for those two requirements.

Might cut the intial investment a bit but still leaves you with the mod/con for your home heating needs.

Just an idea.

Mark H

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rumn8r

Perry,

Thanks for all your comments. It took me a while to digest but I think I understand now.

rumn8r

That won't work because my total heat loss for the house and pool room exceeds the heat output of the largest Vitodens. So I would still need two Vitodens boilers plus a non mod con for the pool and indirect hot water. I'd like to figure out how to supply all my consumers with two Vitodens boilers if possible. It seems that the pool heat exchanger is the big problem here because the indirect hot water doesn't require greater capacity and is controlled by the Vitodens. Although heat to the pool contributes to satisfy the heat loss in the pool room, if I set back the pool heat/ pool room temperature on weekdays to save energy (not enough time to swim) and bring it up on weekends, I will need extra boiler capacity. I might have to abandon that possibility for control and just keep the pool at a constant 82°F and the pool room temperature 84° F (2° F above to minimize humidity and cooling due to evaporation). In that case, I think the mod con would be a great fit for heating the pool water fro what I have read.